MarkWest Energy Partners, L.P. has signed two major agreements related to its ongoing development of the Marcellus Shale.

The first is a long-term fee-based arrangement with affiliates of Chesapeake Energy Corp. to expand its Marcellus gas processing capacity to support Chesapeake’s rapidly growing rich natural gas production in a 185-square-mile dedication area which includes portions of Brooke, Ohio, and Marshall counties in northern West Virginia and Washington County in southwestern Pennsylvania.

The second agreement is a long-term fee-based arrangement with Antero Resources Appalachian Corp. to install gathering facilities in support of Antero’s rapidly growing rich natural gas production in Doddridge and Harrison counties in northern West Virginia.

MarkWest will expand the gas processing capacity at its Majorsville, WV processing complex from 670 MMcf/d to 1.1 Bcf/d. It will construct two 200 MMcf/d cryogenic gas plants that are expected to be completed in late 2013 and mid 2014.

The gathering facility expansion to support Antero Resources will expand MarkWest’s existing gathering operations in southwestern Pennsylvania into a key part of West Virginia and includes the installation of two large high-pressure gathering headers and associated compressor stations in Doddridge and Harrison counties, WV.

The new gathering system will have the capacity to initially deliver more than 300 MMcf/d of Antero’s rich gas to MarkWest’s Sherwood gas processing complex. The first phase will be completed in the third quarter in conjunction with the completion of the 200 MMcf/d Sherwood I processing facility.

The Majorsville expansion will increase MarkWest’s total processing capacity in the rich gas area of the Marcellus to more than 2.1 Bcf/d, essentially all of which is supported by long-term agreements with MarkWest’s producer customers. The NGL recovered at itss four large Marcellus processing complexes in southwest Pennsylvania and northern West Virginia are or will be connected through MarkWest’s NGL gathering system for delivery to its Houston, PA fractionation, storage, and marketing complex.